Electron discharge devices



p 6 A. H. w. BECK 2,764,708

ELECTRON DISCHARGE DEVICES Filed Feb. 15, 1954 Inventor A. H. w. BECK A Home y United States PatentO ELECTRON DISCHARGE DEVICES Arnold Hugh William Beck, London, England, assignor to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application February 15, 1954, Serial No. 410,399

Claims priority, application Great Britain March 20, 1953 7 Claims. (Cl. 313-82) The present invention relates to the construction of electron guns designed for the production of high current density electron beams, using magnetic focussing.

In the construction of electron guns for high current density beams which are to be magnetically focussed, it is desirable, and with some types of beam propagation it may even be necessary, for the cathode to be shielded from the steady axial magnetic focussing field employed with the discharge device of which the gun forms a part. It is also desirable, and sometimes necessary, for the electron beam, on emerging from the gun to be substantially parallel to the desired beam axis, and in U. S. application No. 176,925, filed August 1, 1950, of A. H. W. Beck, a gun is disclosed in which the accelerating anode forms a pole piece for the magnetic focussing arrangement. In the embodiments described in that specification the cathode and focussing electrode were completely enclosed within the pole piece-anode, and contact was made With the remaining part of the magentic circuit through a disc of ferromagnetic material sealed through the envelope of the tube. In the present invention, instead of using a disc, the anode-pole piece forms part of the envelope of the tube and completely surrounds the cathode and focussing electrode. It is a particular object of the present invention to provide such a gun which is simple to manufacture and which can readily be assembled so as to attain the necessary accuracy in relative positioning of the gun electrodes.

According to the present invention there is provided an electron gun for the production of a high current density electron beam to be focussed magnetically, comprising an indirectly heated cathode mounted within a cylindrical focussing electrode having an end cap apertured above the cathode emitting surface and an outer circular flange intermediate its ends, a centrally apertured anode disc of ferromagnetic material abutting against an annular end of, and fitting Within, a generally cylindrical outer sleeve of ferromagnetic material, a cylindrical inner anode sleeve secured to the said disc and surrounding the said focussing electrode, the said inner sleeve having axial portions of different interval diameter to form a shoulder for locating the cathode and focussing electrode with respect to the said anode disc, an annular Washer of insulating material providing an axial location of the said cathode and positioned between the said shoulder and the said outer circular flange, and means for retaining the said anode disc in abutment against the said end of the outer sleeve.

The outer ferromagnetic sleeve is preferably made of an alloy sealing readily to glass, so that a glass closure member, forming a base for the discharge tube, may be sealed at one end and, if desired, a glass envelope portion of the discharge tube may be sealed at the other end of the sleeve. On the other hand it is evident that a gun according to the present invention is Well adapted for use in, for example, an all metal construction of tube envelope.

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An embodiment of the invention will be described with reference to the accompanying drawings, in which:

Fig. 1 shows a view in longitudinal cross-section of an embodiment of the invention and Fig. 2 shows a transverse section taken in the plane IIII of Fig. 1.

The electron gun 1 comprises an indirectly heated cathode 2, having a spherically re-entrant emitting surface. The cathode 2 is supported in a surrounding focussing electrode, generally designated by the numeral 3, formed by a cylindrical member or inner sleeve whose upper end is closed by an end cap 4 which is centrally apertured to disclose, the cathode emitting surface. Intermediate the ends of the focussing electrode, its end cap 4 carries an outer circular flange 5 and, in the embodiment shown on the drawing, the lower end of the inner sleeve of electrode 4 carries a further circular flange 6. During operation the focussing electrode 3 is maintained at cathode potential and may, therefore, be connected internally thereto. The anode of the gun comprises a centrally apertured anode disc 7 of ferromagnetic material to which is sweated a cylindrical inner anode sleeve 8 surrounding the focussing electrode3 and having axial portions of different internal diameter, so as to form a shoulder 9. An annular washer 10, of insulating material such as mica, fits closely about the upper end of the focussing electrode 3, and seats against the shoulder 9, the outer circular flange 5 of the focussing electrode abutting against its under side. In this Way the emitting surface of the cathode 2 is located centrally within the inner-anode sleeve and at the correct axial distance from the anode disc 7. The disc 7 contains a central aperture 11, whose sides are preferably bevelled as shown.

The thickness of the anode disc 7 and the relative dispositions of the cathode focussing electrode and anode are preferably such that, under operating conditions a fully space charge limited current is drawn from the cathode and the beam is focussedthrough the anode aperture 11 so as to be substantially parallel to the longitudinal axis of the gun just after its exit from the aperture. If, as would almost invariably be the case, with such an arrangement, the cathode emitting-surface is concave, as illustrated in the drawing, the gun should be dimensioned in the manner disclosed in my aforementioned U. S. application No. 176,925. The gun is then particularly suitable for launching an electron beam in which the electrons move with Brillouin type flow. Referred to a cylindrical solid beam (as distinct from an annular beam) Brillouin flow is characterised by all the electrons having the same axial velocity and each moving, by virtue of a steady axial magnetic field, in a helix of constant radius about the beam axis with the same angular velocity, so that the motion of the beam is like that of a solid rod rotating about and moving along its axis. With a cathode whose emitting surface is concave when viewed from the direction of propagation of the beam, such flow demands that the cathode should be screened from the influence of the axial magnetic field. In the present invention this is achieved by the anode disc 7 and outer sleeve 12 being of ferromagnetic material. It is known that to improve the magnetic screening of an enclosure it is preferable to use several spaced screens of magnetic material rather than to have a single screen whose thickness is the sum of the thickness of the individual screens. Consequently, it is preferred that the inner anode sleeve 8 should also be constructed of ferromagnetic material. In use the anode disc 7 forms one pole piece of the magnetic focussing arrangement, yoke members of which may conveniently be clamped about the outer sleeve 12. The yoke members still further improve the magnetic screening of the cathode.

While an electron gun according to the present invention is well adapted to form part of an all metal discharge tube construction, some convenient means being provided for retaining pressure between the anode disc 7 and the annular ends 13 of the outer sleeve 12, and also between the circular flange of the focussing electrode, the washer 10 and the shoulder 9 on the inner anode sleeve 8, in the embodiment illustrated in the drawing the outer member 12 is constructed of an alloy sealing readily to glass for example, the alloys commercially known as Cinseal (reg. trademark) or that sold as 50:50 Nickel-Iron Alloy and a glass closure member 15 is sealed to the lower end of the outer sleeve 12. The closure member 15 has a conventional glass base 16 carrying pins 17 sealed therethrough. A plurality of rods 18 are secured to the flange 6 at one end and to convenient ones of the pins 17 at the other to maintain the required axial pressure between the various gun electrodes. In the embodiment iliustrated a glass envelope member 19 of the discharge device, of which the gun forms a part, is butt-sealed to the annular end of 13 of the outer cylinder 12.

By virtue of the fact that the major portions of the electron gun are a sliding fit one within the other, an electron gun according to the invention is easy to manufacture and assemble and the necessary critical spacings between the electrodes can readily be obtained with accuracy.

While the principles of the invention have been described above in connection with specific embodiments, and particular modifications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

What I claim is:

1. An electron gun for the production of a high current density electron beam to be focussed magnetically, comprising an indirectly heated cathode mounted within a cylindrical focussing electrode having an end cap apertured above the cathode emitting surface and an outer circular flange intermediate its ends, a centrally apertured anode disc of ferromagnetic material abutting against an annular end of, and fitting within, a generally cylindrical outer sleeve of ferromagnetic material, a cylindrical inner anode sleeve secured to the said disc and surrounding the said focussing electrode, the said inner sleeve having axial portions of different internal diameter to form a shoulder for locating the cathode and focussing electrode with respect to the said anode disc, an annular washer of insulating material providing an axial location of the said cathode and positioned between the said shoulder and the said outer circular flange, and means for retaining the said flange in abutment against the said annular Washer and the washer against the said shoulder, and for retaining the said anode disc in abutment against the said end of the outer sleeve.

2. An electron gun according to claim 1 in which the said outer sleeve is of a material sealing readily to glass and in which a glass closure member is sealed to the sleeve remote from the said annular end.

3. An electron gun according to claim 2 in which the said means for retaining the anode disc in abutment against the said outer sleeve annular end, together with the said shoulder, washer and outer circular flange in respective abutment; comprises a plurality of rods each of which is secured at one end to the said focussing cylinder and at the other end to the said glass closure member.

4. An electron gun according to claim 3 in which the said rods are each sealed at the said outer end to a valve pin sealed in the base of the said closure member.

5. In an electron discharge device an electron gun according to claim 1 in which the said outer sleeve is of a material sealing readily to glass, a cylindrical glass envelope portion of the said discharge device being buttsealed to the said annular end of the outer sleeve.

6. An electron gun according to claim 1 in which the said inner anode sleeve is of ferromagnetic material.

7. An electron gun according to claim 1 in which the anode disc is so dimensioned and the cathode and focussing electrodes are so positioned relative thereto that under operating conditions a space charge limited electron beam is projected from the gun substantially parallel to the beam axis slightly beyond the exit from the aperture of the anode disc.

References Cited in the file of this patent UNITED STATES PATENTS 

